Plastic scintillator with effective pulse shape discrimination for neutron and gamma detection

ABSTRACT

In one embodiment, a scintillator material includes a polymer matrix; and a primary dye in the polymer matrix, the primary dye being a fluorescent dye, the primary dye being present in an amount of 5 wt % or more; wherein the scintillator material exhibits an optical response signature for neutrons that is different than an optical response signature for gamma rays. In another embodiment, a scintillator material includes a polymer matrix comprising at least one of: polyvinyl xylene (PVX); polyvinyl diphenyl; and polyvinyl tetrahydronaphthalene; and a primary dye in the polymer matrix, the primary dye being a fluorescent dye, the primary dye being present in an amount greater than 10 wt %. A total loading of dye in the scintillator material is sufficient to cause the scintillator material to exhibit a pulse-shape discrimination (PSD) figure of merit (FOM) of about at least 2.0.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/437,836, filed Apr. 2, 2012 and entitled “Plastic Scintillator withEffective Pulse Shape Discrimination for Neutron and Gamma Detection,”which claims priority to provisional U.S. Patent Application No.61/476,071, filed on Apr. 15, 2011, to each of which priority is claimedand each of which is herein incorporated by reference.

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Livermore National Laboratory.

FIELD OF THE INVENTION

The present invention relates to spectroscopy, and more particularspectroscopy materials, systems and methods.

BACKGROUND

Radioactive materials are often detected and identified by measuringgamma-rays and/or neutrons emitted from the materials. The energy ofgamma-rays is specific to that particular material and acts as a “fingerprint” to identify the material. Similarly, neutron energy is particularto the material, and may be used to identify the material. Of very highvalue are detectors capable of identifying the distinctivetime-correlated signatures corresponding to neutrons and gammas emittedby fissioning material from within a background of uncorrelated naturalradiation. A detector capable of distinguishing neutrons from gammas, aswell as offering a fast response time typically has better capabilityfor detecting the distinctive time-correlated events indicative of thepresence of fissioning nuclei.

The ability to detect gamma rays and/or neutrons is a vital tool formany areas of research. Gamma-ray/neutron detectors allow scientists tostudy celestial phenomena and diagnose medical diseases, and they havebeen used to determine the yield in an underground nuclear test. Today,these detectors are important tools for homeland security, helping thenation confront new security challenges. The nuclear non-proliferationmission requires detectors capable of identifying diversion of orsmuggling of nuclear materials. Government agencies need detectors forscenarios in which a terrorist might use radioactive materials tofashion a destructive device targeted against civilians, structures, ornational events. To better detect and prevent nuclear incidents, theDepartment of Energy (DOE) and the Department of Homeland Security (DHS)are funding projects to develop a suite of detection systems that cansearch for radioactive sources in different environments.

One particularly useful type of radiation detection, pulse shapediscrimination (PSD), which is exhibited by some organic scintillators,involves subtle physical phenomena which give rise to the delayedluminescence characteristic of neutrons, providing a means ofdistinguishing neutrons from the preponderance of prompt luminescencearising from background gamma interactions. The mechanism by which thisoccurs begins with the excitation process which produces excited singlet(S1) and excited triplet (T1) states nonradiatively relaxes to theconfiguration, as shown in FIG. 1. In FIG. 1, the basic physicalprocesses leading to the delayed fluorescence characteristic of neutronexcitation of organics with phenyl groups is shown.

Since the triplet is known to be mobile in some compounds, the energymigrates until the collision of two triplets collide and experience anAuger upconversion process, shown as Equation 1:

T₁+T₁→S₀+S₁   Equation 1

In Equation 1, T1 is a triplet, S₀ is the ground state, and S₁ is afirst excited state. Finally, the delayed singlet emission occurs with adecay rate characteristic of the migration rate and concentration of thetriplet population, which is represented as Equation 2:

S₁→S₀+hv   Equation 2

In Equation 2, hv is fluorescence, while S₀ is the ground state and S₁is a first excited state. The enhanced level of delayed emission forneutrons arises from the short range of the energetic protons producedfrom neutron collisions (thereby yielding a high concentration oftriplets), compared to the longer range of the electrons from the gammainteractions. The resulting higher concentration of triplets fromneutrons, compared to gamma interactions, leads to the functionality ofPSD. The observation of PSD is believed to be in part related to thebenzene ring structure, allowing for the migration of triplet energy.

FIG. 2A shows a typical plot of logarithmic population versus lineartime (ns) for stilbene. Population is the singlet excited statepopulation, which is proportional to the output of light from a testcrystal under examination, in this case a stilbene crystal, after thecrystal it is excited by high energy radiation. As can be seen from theplot, some light is produced by the crystal almost immediately, referredto as prompt luminescence, and other light is produced from the crystalover a period of time, referred to as delayed luminescence. Generally,the plot for each type of radiation will have a steep component 202 anda tail component 204, where the differentiation point 206 between thetwo is defined in the region where the slope of the line changesdramatically. In this example, the steep component 202, tail component204, and differentiation point 206 for the Neutron curve is labeled.Note that the steep component, tail component, and differentiation pointfor the Gamma curve is different for stilbene, and other compounds whichpossess good PSD properties. Compounds which do not possess good PSDproperties will generally not have substantial differences in the curvesplotted for Gamma and Neutron radiation. The upper line in the plotshown in FIG. 2A is a Neutron-induced scintillation pulse shape, whilethe lower line is a Gamma-induced scintillation pulse shape. As can beseen, stilbene is able to differentiate between the Neutron and Gammapulse shapes, and produces noticeably different luminescence decaylineshapes for each radiation type. However, not every compound has thisability to separate between Gamma and Neutron pulse shapes, andtherefore compounds which do are very useful for PSD, as Gamma andNeutron luminescence decay plots have different pulse shapes for thesecompounds.

Once the population versus time plot has been determined for each testcrystal under examination, if it appears that there is PSD for thecrystal type, the area (Q_(S)) under the tail component of the curve foreach type of radiation is calculated, along with the area (Q_(F)) underthe entire line for each type of radiation. By dividing the total area(Q_(F)) into the tail area (Q_(S)), a scatterplot of the ratio of chargeversus the pulse height can be produced, as shown in FIG. 2B forstilbene. FIG. 2B shows a plot of the ratio of charge (Q_(S)/Q_(F))versus the pulse height, which correlates to an output of a lightdetector, such as a photomultiplier tube. The x-axis represents thepulse height, which is proportional to the energy of the event. Gammaevents correspond to light produced by Compton electrons generated inthe detector material. Neutron events correspond to proton recoils inthe detector material; lower energy proton recoil events correspond to“glancing angle” interactions between the neutron and proton in thedetector material, while a high energy “knock-on” interaction between aneutron and a proton will produce a higher energy event.

Referring to FIG. 2B, at hv equal to about 1600V, conventionalscintillators utilizing stilbene exhibit a neutron-to-gamma (n°/γ)separation S of about 0.132. The greater the separation S ofneutron-to-gamma, the better PSD performance can be expected.

It is with these scatter plots that good PSD separation can bedetermined, which is defined as PSD separation, S, which is the gapbetween the mean ratio of charge (Q_(S)/Q_(F)) for Gamma and the meanratio of charge (Q_(S)/Q_(F)) for Neutron taken over an extended periodof time. The higher this separation, S, is, the better the compound isat PSD separation.

It is generally accepted in the prior art that stilbene offers good PSD.However, stilbene, generally grown from melt, is difficult to obtain.Therefore, a number of other organic molecules have been examined.Unfortunately, most research in this area has concluded that many knownliquid and solid materials, including many compounds having benzenerings, do not possess PSD properties comparable to single-crystalstilbene. Despite the difficulty in identifying compounds with suitablePSD properties, the inventors previously succeeded in demonstratingseveral exemplary compounds with suitable PSD properties and capable ofbeing grown from solution, including 1-1-4-4-tetraphenyl-1-3-butadiene;2-fluorobiphenyl-4-carboxylic acid; 4-biphenylcarboxylic acid;9-10-diphenylanthracene; 9-phenylanthracene; 1-3-5-triphenylbenzene;m-terphenyl; bis-MSB; p-terphenyl; diphenylacetylene;2-5-diphenyoxazole; 4-benzylbiphenyl; biphenyl; 4-methoxybiphenyl;n-phenylanthranilic acid; and 1-4-diphenyl-1-3-butadiene.

Moreover, crystals such as stilbene, generally grown from melt, aredifficult to obtain. Therefore, organic liquid scintillator cocktailscomprised of an aromatic solvent, such as toluene, a primary and asecondary fluor, have been developed and are commercially available,however, liquid scintillators do not exhibit PSD properties comparable tsingle-crystal stilbene, and are also hazardous to field, because thesecompounds typically include flammable, toxic, and otherwise hazardousmaterials that limit application to sensitive environments such asaviation, military applications, medical applications, and etc.Moreover, the above crystals, especially when grown from solution, tendto be relatively fragile, making safe and efficient transport difficult.

Accordingly, it would be beneficial to provide organic materialscomparable to or better than stilbene in relation to PSD properties forneutron radiation detection, but in a form that is easier to fabricateinto large monoliths which are durable, and which do not introducehazardous material into the radiation detection process.

SUMMARY

In one general embodiment, a scintillator material includes a polymermatrix; and a primary dye in the polymer matrix, the primary dye being afluorescent dye, the primary dye being present in an amount of 5 wt % ormore; wherein the scintillator material exhibits an optical responsesignature for neutrons that is different than an optical responsesignature for gamma rays.

In another general embodiment, a scintillator material includes apolymer matrix; and a primary dye in the polymer matrix, the primary dyebeing a fluorescent dye, the primary dye being present in an amountgreater than 10 wt %.

Other aspects and embodiments of the present invention will becomeapparent from the following detailed description, which, when taken inconjunction with the drawings, illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a mechanism for delayed photoluminescence according to theprior art.

FIG. 2A shows a plot of Population versus Time for stilbene according tothe prior art.

FIG. 2B shows a plot illustrating PSD separation of stilbene accordingto the prior rt.

FIG. 3 shows a simplified schematic layout of a system that may usecrystals described herein, according to one embodiment.

FIG. 4A shows a comparative graphical representation of gamma/neutronseparation plotted according to charge ratio versus pulse height for anorganic plastic scintillator and stilbene, according to one embodiment.

FIG. 4B shows a comparative graphical representation of integratedgamma/neutron separation plotted for an organic plastic scintillatorwith FOM 3 and an Eljen liquid with FOM 2.21, according to oneembodiment.

FIG. 5 shows a comparative graphical representation of integratedgamma/neutron separation plots for organic plastic scintillatorsutilizing different phases and/or concentrations of 2,5-diphenyl oxazole(PPO) and diphenyl anthracene (DPA), according to various embodiments.

FIG. 6 shows a plot comparing integrated gamma/neutron separation forscintillator systems including a plastic scintillator and a primaryfluor, a plastic scintillator, a primary fluor and a secondary fluor, anEljen liquid, and a stilbene crystal, according to various embodiments.

FIG. 7A shows a plot of signal intensity versus pulse integral for anorganic plastic scintillator employing 30% 2,5-diphenyl oxazole (PPO) asa primary fluor versus an organic plastic scintillator employing 30%2,5-diphenl oxazole (PPO) as a primary fluor and 0.2% diphenylanthracene (DPA) as a secondary fluor.

FIG. 7B shows a plot of photoluminescence intensity versus wavelengthfor an organic plastic scintillator employing 30% 2,5-diphenyl oxazole(PPO) as a primary fluor versus a scintillator employing 30%2,5-diphenvl oxazole (PPO) as a primary fluor and 0.2% diphenylanthracene (DPA) as a secondary fluor.

FIG. 8 shows a plot of PSD characteristics versus fluor concentrationfor PPO and DPA solubilized in xylene solution, according to oneembodiment.

FIG. 9 depicts experimental PSD patterns showing increase ofneutron-gamma peak separation at increasing PPO concentration in apolyvinyl toluene (PVT) polymer matrix, according to one embodiment.

FIG. 10 is a plot depicting general dependence of neutron-gammaseparation measured in the whole range of concentrations up to the limitof the PPO solubility.

FIG. 11 is a flowchart of a method, according to one embodiment.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating thegeneral principles of the present invention and is not meant to limitthe inventive concepts claimed herein. Further, particular featuresdescribed herein can be used in combination with other describedfeatures in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be giventheir broadest possible interpretation including meanings implied fromthe specification as well as meanings understood by those skilled in theart and/or as defined in dictionaries, treatises, etc.

It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” include pluralreferents unless otherwise specified. The term “about” as used hereinrefers to ±10% of the denoted value, unless otherwise noted herein.

The description herein is presented to enable any person skilled in theart to make and use the invention and is provided in the context ofparticular applications of the invention and their requirements. Variousmodifications to the disclosed embodiments will be readily apparent tothose skilled in the art upon reading the present disclosure, includingcombining features from various embodiment to create additional and/oralternative embodiments thereof.

Moreover, the general principles defined herein may be applied to otherembodiments and applications without departing from the spirit and scopeof the present invention. Thus, the present invention is not intended tobe limited to the embodiments shown, but is to be accorded the widestscope consistent with the principles and features disclosed herein.

Unless otherwise noted herein, all percentage values are to beunderstood as percentage by weight (wt %) Moreover, all percentages byweight are to be understood as disclosed in an amount relative to thebulk weight of an organic plastic scintillator material, in variousapproaches.

The following description describes several embodiments relating to theuse of the fabrication of polymer scintillator materials withdistinctively different scintillation pulse shapes resulting fromneutron and gamma excitation, respectively.

Traditionally, gamma excitation in the conventional organic plasticscintillator materials using conventional fluor concentrations in therange of about 0.1-5 wt % fluor have exhibited low excitation density,and weak delayed luminescence. Thus, the ability to distinguishhigh-energy neutron radiation from gamma radiation has previously beenachievable only with liquid scintillators, which are difficult to field,and organic single crystals, which can be fragile and difficult toproduce in very large volumes.

Accordingly, the inventive approaches detailed in the presentdisclosures have not been demonstrated in the past, presumably sincevery few fluors are soluble at >5 wt % in a polymer matrix. Indeed, thetraditional polymer scintillator materials have been limited toincluding fluors in concentrations which demonstrably lack PSDcharacteristics, discouragingly suggesting that organic plastic polymermaterials would be unsuitable for use as a scintillator material capableof exhibiting PSD characteristics.

However, the inventors of the presently disclosed scintillator havesurprisingly discovered that in some embodiments, loading an organicplastic polymer material with high concentrations of fluors, (≧10 wt %)results in sufficient migration of triplet excitation within the or althat the higher excitation density produced by neutron-induced protonrecoil results in delayed singlet luminescence from triplet-tripletannihilation. Without wishing to be bound to any particular theory, itis speculated that loading an organic plastic scintillator material with≧10 wt % of a primary fluor and ≦1 wt % of a secondary fluor with betterspectral match to standard photomultipliers may produce an even moreeffective pulse shape discrimination material. Thus, the use of plasticscintillator material for this application enables passive detection ofhigh-energy neutron radiation as distinguishable from gamma radiation,as well as active interrogation methods, according to variousapproaches.

In particular, recent studies conducted with organic crystals showedthat the main reason for the absence of PSD in mixed systems resultsfrom the excitation traps formed by a lower-band-gap fluorescentimpurity (fluor) present in the host material (solvent) at lowconcentration. Moreover, increasing the concentration of the fluor canprovide conditions suitable for formation of a network for excitationenergy migration and triplet annihilation, and may lead to theappearance of PSD comparable to that typical for pure single crystals ofthe fluor, in various approaches.

The present disclosure introduces the results of studies conducted withmixed liquid and plastic scintillating systems. Analysis of the resultsshows that explanations of the conditions leading to the formation ofPSD in crystals and liquids can be similarly applied to the mixedplastic systems. The properties of the exemplary plastics scintillatorsfabricated with efficient neutron/gamma discrimination are discussed incomparison with commercially available liquid and single crystal organicscintillators.

In one general embodiment, a scintillator material includes a polymermatrix; and a primary dye in the polymer, the primary dye being afluorescent dye, the primary dye being present in an amount of 5 wt % ormore; wherei: the scintillator material exhibits an optional responsesignature for neutrons that is different than an optical responsesignature for gamma rays.

In another general embodiment, a scintillator material includes apolymer and a primary dye in the polymer matrix, the primary dye being afluorescent dye, the primary dye being present in an amount greater than10 wt %.

General Scintillator-Based Radiation Detector System

FIG. 3 depicts a simplified spectroscopy system according to oneembodiment The system 300 comprises a scintillator material 302, such asof a type described herein, and which is referred to hereininterchangeably as a scintillator. The system 300 also includes aphotodetector 304, such as a photomultiplier tube or other device knownin the art, which can detect light emitted from the scintillator 302,and detect the response of the material to at least one of neutron andgamma ray irradiation.

The scintillator 302 produces light pulses upon occurrence of an event,such as a neutron, a gamma ray, or other radiation engaging thescintillator 302. As the gamma ray, for example, traverses thescintillator 302, photons are released, appearing as light pulsesemitted from the scintillator 302. The light pulses are detected by thephotodetector 304 and transduced into electrical signals that correspondto the pulses. The type of radiation can then be determined by analyzingthe integral of the light pulses and thereby identifying the gamma rayenergy absorbed by the scintillator.

In some embodiments, the system 300 may be, further comprise, or becoupleable/coupled to, a processing device 306 for processing pulse acesoutput by the photodetector 304. In other embodiments, the system 300may include a processing device that receives data from a photodetectorthat is not permanently coupled to the processing device. Illustrativeprocessing devices include microprocessors, field programmable gatearrays (FPGAs), application specific integrated circuits (ASICs),computers, etc.

The result of the processing may be output and/or stored. For example,the result may be displayed on a display device 308 in any form, such asin a histogram or derivative thereof.

The program environment in which one embodiment of the invention may beexecuted illustratively incorporates one or more general-purposecomputers or special-purpose devices such hand-held computers. Detailsof such devices (e.g., processor, memory, data storage, input and outputdevices are well known and are omitted for the sake of clarity.

It should also be understood that the techniques of the presentinvention might he implemented using a variety of technologies. Forexample, the methods described herein may be implemented in softwarerunning on a computer system, or implemented in hardware utilizing oneor more processors and logic (hardware and/or software) for performingoperations of the method, application specific integrated circuits,programmable logic devices such as Field Programmable Gate Arrays(FPGAs), and/or various combinations thereof. In particular, methodsdescribed herein may be implemented by a series of computer-executableinstructions residing on a storage medium such as a physical (e.g.,non-transitory) computer-readable medium. In addition, although specificembodiments of the invention may employ object-oriented softwareprogramming concepts, the invention is not so limited and is easilyadapted to employ other forms of directing the operation of a computer.

Portions of the invention can also be provided in the form of a computerprogram product comprising a physical computer readable medium havingcomputer code thereon. A computer readable medium can include anyphysical medium capable of storing computer code thereon for use by acomputer, including optical disks such as read only and writeable CD andDVD, magnetic memory or medium (e.g., hard disk drive), semiconductormemory (e.g., FLASH memory and other portable memory cards, etc.), etc.

Polymer

The inventive organic plastic scintillator system as encompassed by thepresent disclosures may include any suitable polymer as the plasticcomponent. Particularly suitable re rigid, durable, transparentplastics, possessing an aromatic structure consisting of pi-conjugatedrings, and capable of supporting high concentrations of primary,secondary, tertiary, and etc. fluors, e.g. with a total concentration inthe range of about 5-75 wt % fluor, according to some embodiments. In apreferred embodiment, the organic plastic component may include apolymer comprising polyvinyltoluene (PVT). In other embodiments, similarpolymers may be utilized, such as polystyrene (PS), polyvinyl xylene(PVX), polymethyl, 2,4-dimethyl, 2,4,5-trimethyl styrenes, polyvinyldiphenyl, polyvinyl naphthalene, polyvinyl tetrahydronaphthalenepolymers, other complex aromatic polymers, and certain non-aromaticpolymers capable of solubilizing high fluor concentrations, etc. aswould be understood by one having ordinary skill in the art upon readingthe present descriptions.

As described herein, suitable polymers may be preferably at least 95%light transmissive in a wavelength of interest, e.g., a wavelengthemitted by one or more fluors present in the organic plasticscintillator system, in some embodiments.

Moreover, the polymer may be provided as a liquid polymer matrix, anon-liquid polymer matrix, a dry powder, etc. as would be understood byone having ordinary skill in the art upon reading the presentdescriptions. Moreover, in various approaches the polymer matrix mayinclude aromatic functional groups, such as phenyl groups, among others.

Fluors

Primary fluors suitable for use in the presently disclosed scintillatorsystem include any fluor as known in the art and capable of exhibitingcharacteristics of pulse-shape discrimination as described herein.Moreover, the primary fluor of the exemplary organic plasticscintillator system is present in high concentration, e.g. about 5 wt %or more, in one embodiment. In preferred embodiments, the primary dyemay be present in an amount of 20 wt % or more, and in particularlypreferred embodiments, the primary dye may be present in an amountranging from about 20 wt % to about 75 wt % or an amount ranging fromabout 30 wt % to about 75 wt %. As discussed herein, the concentrationsof fluor are described relative to a weight of the bulk scintillatormaterial, in various embodiments.

In one particular embodiment, a scintillator system may include apolymer matrix and a primary fluor disposed in the polymer matrix.Moreover, the primary fluor may be a fluorescent dye present in anamount of 5 wt % or more, and such fluorescent dye results in thescintillator material exhibiting an optical response signature forneutrons that is different than an optical response signature for gammarays.

Accordingly, where primary fluors are present in high concentration inthe exemplary organic plastic scintillator system, a corollary principleis that the solubility of the fluor in the polymer is preferably high.In one embodiment, for example, the polymer may be characterized byhaving a solubility of about 5 wt % or more with respect to particularfluor.

Moreover, in some approaches the primary fluor may include multipledyes. In further approaches the primary fluor may include multiplefluorescent dyes.

Moreover still, primary fluor may be incorporated into the polymeraccording to any suitable mechanism. For example, in one embodiment theprimary fluor may be suspended in the polymer matrix and dispersed to anapproximately uniform distribution. In other embodiments, the primaryfluor may be crosslinked to the polymer matrix. In still otherembodiments, the primary fluor may be copolymerized with the polymermatrix, and/or with another component of the polymer matrix, etc. aswould be understood by one having ordinary skill in the art upon readingthe present descriptions. Of course, other arrangements of fluor andpolymer matrix may be utilized without departing from the scope of thepresent descriptions.

The secondary fluor of the exemplary plastic scintillator system ischaracterized by wavelength-shifting qualities, such that in thepresence of another fluor, particularly a primary fluor present in highconcentration in a plastic scintillator system, PSD characteristics ofthe plastic scintillator system with the primary fluor and the secondaryfluor in combination are superior to PSD characteristics of a plasticscintillator system having the same primary fluor exclusively present inhigh concentration, according to one embodiment.

Suitable secondary fluors include any fluor characterized bywavelength-shifting as described herein, and several exemplaryembodiments may utilize secondary fluors such as diphenyl anthracene(DPA), tetraphenyl butadiene (TPB) 1,1,4,4-tetraphenyl-1,3-butadiene,1,4-Bis(5-phenyl-2-oxazolyl)benzene (POPOP),p-bis(o-methylstyryl)benzene,1,4-bis-2-(4-methyl-5-phenyloxazolyl)benzene,2,2′-p-phenylenebis(5-phenoxazole), diphenylstilbene,1,3,5-triaryl-2-pyrazolines,4-(n-butylamino)-2-(4-methoxyphenyl)benzo[b]pyrylium perchlorate, sodiumsalicylate, 1,4-bis(2-methylstyryl)benzene (Bis-MSB),7-dimethylamino-4-methyl-2-quinoline, 7-amino-4-methylcoumarin,4,6-dimethyl-7-ethylamino coumarin, etc. as would be understood by onehaving ordinary skill in the art upon reading the present descriptions.Particularly preferred secondary fluors include DPA, TPB, POPOP, andBis-MSB according to one embodiment.

Regarding the concentration of the secondary fluor, as described hereinthe exemplary organic plastic scintillator system may include secondaryfluor in a low concentration in order to maximize the beneficialwavelength-shifting effects on PSD performance. For example, in oneembodiment the secondary fluor may be present in an amount of about 2 wt% or less. As described herein, secondary fluors may be present in anamount described relative to a weight of the bulk scintillator material.

Particularly impressive PSD values have surprisingly been obtained witha plastic scintillator embodiment including polyvinyl toluene (PVT) with30% 2,5-diphenyl oxazole (PPO) as primary fluor and 0.5% diphenylanthracene (DPA), as secondary fluor, according to one embodiment.Particularly surprising is the solubility of PPO in the PVT polymer,which allows for excellent PSD characteristics described herein.

One preferred embodiment of a PSD plastic can by formed by combining:0.1-1% Benzoyl peroxide (initiator), 30% 2,5-diphenyl oxazole (PPO) asprimary fluor, 0.2-0.5% diphenyl anthracene (DPA), or tetraphenylbutadiene (TPB) as secondary fluor and balance vinyl toluene. Theprocess of creating the plastic corresponding to this embodiment mayinclude: adding the materials listed above to a container in a gloveboxunder an N₂ atmosphere, such as a wide mouth glass jar in one exemplaryembodiment, placing the container in an oven at 80 C, and allowing thejar to sit in the oven undisturbed for four days, after which it iscooled to room temperature in ambient conditions. The resultant polymeris rigid, substantially transparent and offers excellent scintillationproperties for pulse shape discrimination.

Scintillator Fabrication

Various embodiments may employ any known scintillator material withoutdeparting from the scope of the present disclosure. However, severalpreferred approaches for fabricating scintillators with suitable PSDcharacteristics from organic plastic are described below.

In one embodiment, liquid scintillator mixtures were fabricated fromanhydrous p-xylene (>99%), 2,5-diphenyloxazole (PPO, 99%), and9,10-diphenylanthracene (DPA, >98%) in the oxygen-free atmosphere of anitrogen-filled glovebox. PPO was used as received, DPA was stirred for0.5 h in warm acetone, collected by filtration, dried, and stored innitrogen atmosphere prior to sample preparations. The liquid mixtures ofrequired concentrations were transferred into sealed 50 mm×10 mmcylindrical quartz cuvettes and subsequently used for furthermeasurements.

In another approach, plastic scintillator mixtures were fabricated asfollows. Vinyl toluene was filtered through a chromatographic supportmaterial to remove inhibitor prior to polymerization. Filtered vinyltoluene and an initiator, e.g. benzoyl peroxide, were sparged withnitrogen for 40 minutes and stored in sealed containers in a gloveboxrefrigerator at −20 C. To conduct polymerization, required amounts ofPPO and DPA were weighed in a glovebox into 20 mL scintillation vials,initiator (10-30 mg) and vinyl toluene were then added to make up thefinal weight proportions of polymer parts. The vials were tightlysealed, removed from the glovebox, and placed in an oven at 80 C. Twohours later they were shaken to ensure complete mixing, and then held at80 C for a total of 96 hours. After cooling to room temperature theglass was scored and broken with a mallet to remove the barescintillator part.

Of course, the above fabrication methodologies are provided only by wayof example, and organic plastic scintillator systems comprising polymersother than PVT and/or fluors other than DPA/PPO may be fabricated undersimilar conditions, but taking account for slight variations in variousapproaches, e.g. to temperature, incubation time, amount of respectivecomponents, etc. as would be understood by a skilled artisan reading thepresent descriptions.

Furthermore, the present descriptions also encompass methods forfabricating scintillator materials as described herein, as particularlyrepresented by FIG. 11, in various approaches. FIG. 11 depicts a method1100. As will be appreciated by the skilled artisan reading the presentdescriptions, the method 1100 may be performed in any environment,including those depicted in FIGS. 1-10, among others. Regardless ofenvironment, the method 1100 is characterized by operation 1102, where ascintillator precursor mixture is placed in a heating vessel, andsubsequently heated until a polymerization process has completed inoperation 1104. In one embodiment, the scintillator precursor mixture isa combination of about 60-95 wt % of a monomer, about 5-40 wt % of aprimary fluor, and about 0.1 wt % of a free radical initiator. Anymonomer capable of polymerizing and solvating the primary fluor in anamount ranging from about 5-40 wt % may be employed in the exemplaryfabrication process, according to one embodiment and as would beunderstood by one having ordinary skill in the art upon reading thepresent descriptions.

In one particular embodiment, the fabrication process may includecombining about seven grams of vinyl toluene, about 2.94 grams of PPO,and about 0.01 grams of benzoyl peroxide in a heating vessel, mixing thecombination, and heating the mixture at about 80° C. for approximately96 hours.

In another embodiment, the fabrication process may include combiningabout seven grams of vinyl toluene, about 2.94 grams of PPO, about 0.05grams of DPA, and about 0.01 grams of benzoyl peroxide in a heatingvessel, mixing the combination, and heating the mixture at about 80° C.,for approximately 96 hours.

Notably, the solid structure may include any of the structures describedherein, and the primary fluor may include any fluor as described herein,according to some approaches.

Experimental Results: Photoluminescence

Growth and characterization of mixed single crystals led to anunderstanding of the mechanisms of excited state migration andinteraction, prompting an explanation of compositions of polymerscintillator with sufficient fluor to reach the percolation thresholdwhereupon triplet excitation is able to migrate and annihilate.Experiments with complex liquid mixtures lead to findings that highloading with fluors helps improve pulse shape discrimination (PSD) insuch organic scintillators as well. So far, the polymer scintillatorsoffering PSD exhibit a figure of merit (FOM) for PSD of 3, comparedto >4 for certain single crystal organics, such as stilbene, etc. Thisperformance metric is already sufficient to distinguish neutrons fromgammas down to the few hundred keV/gamma equivalent regime, and will bevery useful for non-proliferation, homeland security and safeguardsapplications.

Photoluminescence (PL) spectra were measured under UV excitation using acommercial Spex Fluoromax-2 spectrometer. The scintillation light yieldefficiency was evaluated from the position of the Compton edge in the¹³⁷Cs spectra, in which 480 keVee (electron-equivalent energy) wasdefined by 50% of the Compton edge peak. Neutron detection properties ofsamples were studied using a ²⁵²Cf source shielded with 5.1 cm of lead,which reduced the gamma rates to the same order of magnitude asneutrons, to irradiate liquid or plastic samples coupled to HamamatsuR6231-100-SEL photomultiplier tube (PMT). The signals collected at theMIT anode were recorded using a 14-bit high-resolution waveformCompuScope 14200 digitizer with a sampling rate of 200 MS/s, for offlineanalysis.

The ability of scintillators to discriminate between the neutrons andgamma rays emitted from the ²⁵²Cf source was evaluated using the chargeintegration method. In particular, waveforms were numerically integratedover two time intervals: Δt_(Total) and a subinterval Δt_(Tail),corresponding to the total charge (Q_(Total)) and the delayed component(Q_(Tail)) of the signal, respectively. The value of the ratio of chargeR=Q_(Tail)/Q_(Total) for the two time intervals indicated whether theconsidered event was likely produced by a neutron (high R value) or agamma ray (small R value). Quantitative evaluation of PSD was made usingFigures of Merit (FOM) as represented in Equation 3, where S is theseparation between gamma and neutron peaks, and δ_(gamma) andδ_(neutron) are the full width at half maximum (FWHM) of thecorresponding peaks, as shown in FIG. 5, according to one embodiment.

FOM=S/(δ_(gamma)+δ_(neutron))   Equation 3

The experimental separation S was calculated as a difference between themean delayed light fraction, (Q_(Tail)/Q_(Total)) for neutrons andgammas taken as a normal distribution in PSD over a specified energyrange. In total 40000 events were collected for each scintillatorsample, with approximately 20% of the statistics used for FOMcalculation in the energy range near the Compton edge.

A reasonable definition for well separated Gaussian distributions ofsimilar population sizes is shown in Equation 4, below, where σ is thestandard deviation for each corresponding peak.

σ>3(σ_(gamma)+σ_(neutron))   Equation 4

Noting that where FWHM≈2.36, a reference parameterFOM≧3(σ_(gamma)+σ_(neutron))/2.36(σ_(gamma)+σ_(neutron)) was used todefine efficient PSD in the tested samples. The experimentallydetermined efficiency value was ≈1.27

One characteristic of the exemplary scintillator material describedherein is that the system exhibits an optical response signature forneutrons that is different than an optical response signature for gammarays, according to one embodiment. In particular, the neutron opticalresponse signature may be in the range of about 600-800 keV gammaequivalent, according to one embodiment.

FIGS. 4A-4B, show examples of PSD patterns measured in some organicswith phenyl groups according to several embodiments. FIG. 4A shows acomparison of the charge ratio Q_(O)/Q_(F) as exhibited by oneembodiment of an organic plastic scintillator as described in thepresent disclosures, and a scintillator utilizing a stilbene crystal. Ascan be seen from the figure, the inventors have successfully createdorganic plastic scintillator embodiments that exhibit similar PSDcharacteristics as single crystal systems employing stilbene as thescintillating material. Accordingly, it is possible to generate a morerigid and durable, scintillator material from organic plastic whileretaining the advantageous neutron and gamma radiation separationcharacteristics of more expensive and fragile alternatives such asliquid and single crystal scintillators.

FIG. 4B depicts comparative FOM readings taken from an exemplary organicplastic scintillator exhibiting PSD as described herein and according toone embodiment as compared to a FOM reading of a LLNL Eljen liquidscintillator, according to one embodiment. When comparing FOM readingsfrom FIG. 4B, both of which used test samples of approximately the samevolume, a clear improvement in the FOM levels is apparent. Particularly,the FIG. 4B illustrates a FOM reading of 3 taken from a LLNL plasticscintillator, according to one embodiment. This FOM reading shows PSDapproaching the level of typical liquid and single crystal organicscintillators, thus improving neutron/gamma discrimination properties ofboth liquid and plastic scintillators in the embodiments shown, amongothers within the scope of the present descriptions.

The discovery that organic plastic scintillators are suitablealternatives for more dangerous, expensive, burdensome, etc. systemssuch as single crystal scintillators and liquid scintillators led theinventors to experiment with a variety of polymer and fluor candidates.With reference to FIG. 5, several exemplary embodiments of organicplastic scintillators as described herein are depicted according to FOMperformance of various embodiments including different fluor(s) in arange of concentrations, in some approaches.

As shown in FIG. 5, the top row of graphs represent measurements thatwere taken from liquid scintillator systems comprising diphenylanthracene (DPA) as a fluor. Conversely, the bottom row of graphsrepresent measurements that were taken from scintillator systemscomprising 2,5-diphenyl oxazole (PPO) as a fluor. Each fluor exhibitscharacteristic FOM as a crystal, shown in FIG. 5 as FOM=4.42 for DPAcrystal and FOM=1.76 for PPO crystal powder, respectively.

Moreover, as can be seen from FIG. 5, fluor FOM generally decreases as afunction of concentration in solution. For example, a solution of about2.2% DPA yields a FOM value of about 3.79, according to one embodiment,while a solution of about 0.05% DPA exhibits a FOM value of about 1.12,according to another embodiment, and a solution of about 0.01% DPAexhibits a FOM value of about 0.58, according to yet another embodiment.

Similarly, liquid solutions of PPO yield FOM values that decrease as afunction of PPO concentration. For example, as shown in FIG. 5 asolution of about 37.5% PPO yields a FOM value of approximately 3.26,while a solution of about 0.05% PPO yields a FOM value of about 1.52,and a solution of about 0.01% PPO yields a FOM value of about 0.71,according to several embodiments.

Referring now to FIG. 6, a plot comparing integrated gamma/neutronseparation for scintillator systems including a plastic scintillator anda primary fluor, a plastic scintillator, a primary fluor and a secondaryfluor, an Eljen liquid, and a stilbene crystal is shown, according tovarious embodiments.

Panel A of FIG. 6 depicts PSD characteristics of an organic plasticscintillator system including PPO as a primary fluor, and no secondaryfluor. As can be seen from the figure, this system exhibits a FOM valueof about 2.82, according to one embodiment. Comparatively, panel B showsa similar system comprising an organic plastic scintillator with aprimary fluor including PPO and further including a secondary fluor. Ascan be seen from FIG. 6, this system exhibits superior FOM valuescompared to the similar system lacking a secondary fluor, and in oneembodiment exhibits a FOM value of approximately 3.31.

Further still, Panels C and D of FIG. 6 provide reference points fromwhich to compare the performance of the exemplary organic plasticscintillator systems as described herein against the performance ofliquid and single crystal stilbene scintillator systems, respectively.As can be seen from the figure, the liquid scintillator system shown inPanel C exhibits a FOM value of approximately 3.21, while the singlecrystal stilbene scintillator system shown in Panel D exhibits a FOMvalue of about 4.70, according to some embodiments. Notably, theexemplary organic plastic scintillator system including PPO as a primaryfluor and a secondary fluor outperforms the liquid scintillator systemin terms of FOM value, and approximates the performance of theindustry-standard single crystal stilbene system. Accordingly, highquality PSD characteristics may be imparted to modern scintillatorsystems without incorporating expensive, fragile, and/or hazardousmaterials of the conventional scintillator system.

Referring now to FIGS. 7A and 7B, the comparative effects of includingmultiple fluors in a scintillator crystal are shown, according to oneembodiment.

As may be seen particularly from FIG. 7A, a plastic scintillator having,e.g. 30% w/v PPO as a primary fluor is capable of neutron detection.Moreover, neutron detection may be additionally improved byincorporating a low concentration of a secondary fluor, where thesecondary fluor is characterized by emitting photons of lower energy(longer wavelength) than photons emitted by the primary fluor, accordingto one embodiment.

As shown in FIG. 7B, embodiments of plastic scintillators containingonly the primary fluor PPO exhibit an emission spectrum with a majorpeak near 360 nm. By contrast, embodiments of plastic scintillatorscontaining primary and secondary fluor, such as DPA, exhibit emissionspectra characterized by a peak shift to a longer wavelength (lowerenergy). Moreover, including a secondary fluor also results in greaterluminescence than in systems including only a primary fluor, accordingto one embodiment.

Moreover, including high concentrations of primary fluor improves PSDbehavior, but at higher concentrations disadvantageous self-absorptionof emitted photons traps the light and prevents its emission from thescintillator. In one approach, including a secondary fluor with a loweremission wavelength than the primary fluor shifts the energy of theemitted photon and preventing unfavorable absorption thereof by theprimary fluor molecules. Preferably, secondary fluor may be included insuch low concentration that self-absorption is negligible, therebycircumventing the disadvantages inherent to high concentrations of asingle fluor, according to one embodiment.

Referring now to FIG. 8, the dependence of PSD on fluor concentration isshown as measured in one embodiment including PPO as a primary fluor andDPA as a secondary fluor through the entire range of respectivesolubility in xylene.

For both types of solutions, there is a region of very low fluorconcentrations (<1 μmole/g solution, or ˜0.02 wt. %) with negligiblysmall PSD. Increasing the concentration leads to a gradual enhancementof PSD which, despite the large difference in the solubility,surprisingly exhibits a similar slope for both fluors up to a molecularconcentration of about 10 μmole/g solution. It is further interesting tonote that the separation of the PSD curves unexpectedly occurs at aconcentration corresponding to the maximum light yield for both PPO andDPA (˜10 μmole/g), as particularly shown in panel B of FIG. 8, accordingto one embodiment.

This decrease of the scintillation light efficiency at increasing fluorconcentration (concentration quenching) is ascribed to the formation ofexcimers (S₀S₁). Therefore, the separation of the PSD curves may relateto different kinetics of these processes for different types of themolecules. For example, in one embodiment a rise in PSD to efficientvalues above a concentration threshold as measured in DPA and PPOsolutions is similar to that observed in mixed crystal scintillationsystems, which exhibit PSD behavior according to the simple model ofenergy transfer and triplet-triplet annihilation shown in FIG. 1.

Accordingly, at very small concentrations, excited singlet states of thesolute molecules still produce scintillation light, while excitedtriplets behave more like energy traps, since direct fluorescence fromthe triplets is effectively forbidden.

Meanwhile, at relatively large intermolecular distances and lowprobability of collisions, e.g. in dilute solutions, fluor moleculescannot interact, thus leading to quenched triplet migration,recombination, and a resulting degradation, or even worse, absence ofPSD behavior. At the higher concentrations, increased probability oftriplet-triplet collisions leads to the enhancement of the delayed lightand a rise of the PSD above a certain concentration thresholdcorresponding to the establishment of a continuous network ofinteracting fluor molecules.

Several exemplary organic plastic scintillators with varied fluorconcentration were analyzed, and some results of several exemplaryembodiments are shown in FIG. 9. As evidenced by FIG. 9, a general trendis observable wherein quality of separation between neutron radiationsignals and gamma radiation signals exhibits improved resolution withincreasing primary fluor concentration. For example, as shown in FIG. 9one embodiment of an organic plastic scintillator system including only1 wt % PPO as a primary fluor exhibits no resolution of neutron andgamma radiation signals, respectively, and therefore is incapable ofdiscriminating between the two as required for suitable PSD performance.At about 6 wt % PPO, neutron radiation and gamma radiation signals beginto resolve, but still insufficiently to achieve desirable PSDcharacteristics, according to some embodiments.

In some approaches, true separation between neutron radiation and gammaradiation signals can be seen at primary fluor concentrations of about10 wt %, and improve with increased primary fluor concentration to about15 wt %, as shown in FIG. 9, according to one embodiment. Moreover,separation continues to improve with increasing primary fluorconcentration, and signals are completely distinguishable according toone embodiment including about 30 wt % primary fluor. Of course, furtherincreases in primary fluor concentration may be expected to furtherincrease signal separation and hence PSD performance, but the inventorshave observed that primary fluor concentrations above about 75 wt % mayexhibit inhibitory effects on signal resolution and PSD performance.Accordingly, primary fluor concentrations in a range of about 10 wt % toabout 75 wt % relative to the bulk weight of the scintillator materialare preferred, in some approaches.

FIG. 10 further depicts the generally observed relationship betweenfluor concentration and PSD performance with particular reference tofraction of delayed light for both neutron and gamma radiation,according to one embodiment. As can be seen from FIG. 10, organicplastic scintillators as described herein and utilizing PPO as a primaryfluor exhibit a direct relationship between PPO concentration and amountof delayed light, whether from gamma or neutron radiation. Accordingly,FOM values and corresponding PSD characteristics also improve withincreasing primary fluor concentration, in some embodiments.

Accordingly, the fixed position of molecules in a polymer matrixindicate advantageous performance where the concentration of a fluorrequired for efficient PSD is closer to that in mixed crystals ratherthan in liquid solutions, in various approaches.

Digital Processing for Pulse Shape Discrimination

in one digital processing approach, signals corresponding to subset ofthe events are selected and processed.

Yet another approach includes processing two or more integration windows(e.g., 0-τ₁, τ₁-τ₂), and employing this ratio to deduce a pulse shapediscrimination factor, derived from each individual scintillation pulse.

In any approach, and particularly approaches utilizing digitization asdescribed herein, the exemplary scintillator system employing an organicplastic polymer may further include additional components. In oneembodiment, for example, the exemplary scintillator system may include aprocessor, e.g. for performing a discrimination method for processing anoutput of the photodetector using pulse shape discrimination fordifferentiating responses of the material to the neutron and gamma rayirradiation. In another embodiment, the exemplary scintillator systemmay additionally and/or alternatively include a photodetector, e.g. fordetecting the response of the material to at least one of neutron andgamma ray irradiation. Of course, other components as would beunderstood by the skilled artisan reading the present descriptions maybe included and/or excluded according to various approaches.

In various embodiments, digital processing for pulse shapediscrimination may be performed substantially as described in U.S.patent application Ser. No. 13/024,066, filed Feb. 9, 2011, which isincorporated in its entirety herein by reference.

Applications and Uses

Embodiments of the present invention may be used in a wide variety ofapplications, and potentially any application in which high light yieldand/or pulse shape discrimination between gammas, neutrons, chargedparticles, etc. is useful.

Illustrative uses of various embodiments of the present inventioninclude, but are not limited to, applications requiring radiationdetection. Search, surveillance and monitoring of radioactive materialsare a few such examples. Various embodiments can also be used in thenuclear fuel cycle, homeland security applications, nuclearnon-proliferation, medical imaging, special nuclear material, highenergy physics facilities, etc. Moreover, the figure of merit (FOM)performance metric is already sufficient to distinguish neutrons fromgammas down to the few hundred keV/gamma equivalent regime, and will bevery useful for non-proliferation, homeland security and safeguardsapplications.

Yet other uses include detectors for use in treaty inspections that canmonitor the location of nuclear missile warheads in a nonintrusivemanner. Further uses include implementation in detectors on buoys forcustoms agents at U.S. maritime ports, cargo interrogation systems, andinstruments that emergency response personnel can use to detect orsearch for a clandestine nuclear device. Assessment of radiologicaldispersal devices is another application.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of a preferred embodiment shouldnot be limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A scintillator material comprising: a polymermatrix; and a primary dye in the polymer matrix, the primary dye being afluorescent dye, the primary dye being present in an amount of 5 wt % ormore; and wherein the scintillator material exhibits an optical responsesignature for neutrons that is different than an optical responsesignature for gamma rays.
 2. The scintillator material of claim 1,wherein the primary dye is present in an amount of 50 wt % or more. 3.The scintillator material of claim 1, wherein the primary dye is presentin an amount ranging from about 50 wt % to about 75 wt %.
 4. Thescintillator material of claim 1, wherein the polymer matrix comprisesat least one polymer selected from a group consisting of: polyvinylxylene (PVX); polyvinyl diphenyl; and polyvinyl tetrahydronaphthalene.5. The scintillator material of claim 1, further comprising a secondarydye having a longer emission wavelength than the primary dye.
 6. Thescintillator material of claim 5, wherein the secondary dye comprises1,4-bis(2-methylstyryl)benzene (Bis-MSB); and wherein the secondary dyeis present in an amount of less than 2 wt % and more than 0.2 wt %. 7.The scintillator material of claim 1, wherein the primary dye iscrosslinked to the polymer matrix.
 8. The scintillator material of claim1, wherein the scintillator material is formed into a rigid, solidplastic structure.
 9. The scintillator material of claim 1, wherein theprimary dye includes multiple types of fluorescent dyes.
 10. Thescintillator material of claim 1, further comprising a photodetectorcoupled to the scintillator material, the photodetector being configuredto detect the optical response signature of the scintillator material toat least one of neutron and gamma ray irradiation.
 11. The scintillatormaterial of claim 10, further comprising a processor coupled to thephotodetector, the processor being configured to process an output ofthe photodetector using pulse shape discrimination and differentiateresponses of the scintillator material to the neutron and gamma rayirradiation.
 12. The scintillator material as recited in claim 1,further comprising: a secondary dye in the polymer matrix present in anamount from about 0.2 wt % to about 2 wt %; and an initiator in thepolymer matrix; wherein the secondary dye is characterized by anemission wavelength longer than an emission wavelength of the primarydye; wherein the primary dye is present in an amount ranging from about50 wt % to about 75 wt %; wherein the primary dye is crosslinked to thepolymer matrix; wherein the primary dye includes multiple types offluorescent dyes; wherein a total loading of dye in the scintillatormaterial is sufficient to cause the scintillator material to exhibit apulse-shape discrimination (PSD) figure of merit (FOM) of about at least2.0; wherein the primary dye comprises 2,5-diphenyl oxazole (PPO); andwherein the secondary dye comprises a compound selected from: diphenylanthracene (DPA), tetraphenyl butadiene (TPB),1,4-Bis(5-phenyl-2-oxazolyl)benzene (POPOP) and1,4-bis(2-methylstyryl)benzene (Bis-MSB).
 13. A scintillator materialcomprising: a polymer matrix comprising at least one of: polyvinylxylene (PVX); polyvinyl diphenyl; and polyvinyl tetrahydronaphthalene;and a primary dye in the polymer matrix, the primary dye being afluorescent dye, the primary dye being present in an amount of greaterthan 10 wt %; and wherein a total loading of dye in the scintillatormaterial is sufficient to cause the scintillator material to exhibit apulse-shape discrimination (PSD) figure of merit (FOM) of about at least2.0.
 14. The scintillator material of claim 13, further comprising aninitiator in the polymer matrix.
 15. The scintillator material of claim13, further comprising a secondary dye having a longer wavelength thanthe primary dye, wherein the secondary dye is present in an amount ofless than 2 wt %; and wherein the scintillator material PSD FOM is atleast about 3.0.
 16. The scintillator material of claim 13, wherein theprimary dye is characterized as being either crosslinked orcopolymerized with another component of the polymer matrix.
 17. Thescintillator material of claim 13, wherein the primary dye includesmultiple types of fluorescent dyes.
 18. The scintillator material asrecited in claim 13, wherein the polymer matrix consists of the at leastone of: polyvinyl xylene (PVX); polyvinyl diphenyl; and polyvinyltetrahydronaphthalene.
 19. The scintillator material as recited in claim13, wherein the scintillator material exhibits an optical responsesignature for neutrons that is different than an optical responsesignature for gamma rays; and the scintillator material furthercomprising a photodetector coupled to the scintillator material, thephotodetector being configured to detect the optical response signatureof the scintillator material to at least one of the neutrons and thegamma rays.
 20. The scintillator material as recited in claim 19,further comprising a processor coupled to the photodetector, theprocessor being configured to process an output of the photodetectorusing pulse shape discrimination and differentiate responses of thescintillator material to the neutron and gamma ray irradiation.